Risk and protective factors for self-harm and suicide behaviours among serving and ex-serving personnel of the UK Armed Forces, Canadian Armed Forces, Australian Defence Force and New Zealand Defence Force: A systematic review

Background Self-harm and suicide behaviours are a major public health concern. Several factors are associated with these behaviours among military communities. Identifying these factors may have important implications for policy and clinical services. The aim of this review was to identify the risk and protective factors associated with self-harm and suicide behaviours among serving and ex-serving personnel of the United Kingdom Armed Forces, Canadian Armed Forces, Australian Defence Force and New Zealand Defence Force. Methods A systematic search of seven online databases (PubMed, Web of Science, Embase, Global Health, PsycINFO, PTSDpubs and CINAHL) was conducted alongside cross-referencing, in October 2022. Following an a priori PROSPERO approved protocol (CRD42022348867), papers were independently screened and assessed for quality. Data were synthesised using a narrative approach. Results Overall, 28 papers were included: 13 from Canada, 10 from the United Kingdom, five from Australia and none from New Zealand. Identified risk factors included being single/ex-relationship, early service leavers, shorter length of service (but not necessarily early service leavers), junior ranks, exposure to deployment-related traumatic events, physical and mental health diagnoses, and experience of childhood adversity. Protective factors included being married/in a relationship, higher educational attainment, employment, senior ranks, and higher levels of perceived social support. Conclusion Adequate care and support are a necessity for the military community. Prevention and intervention strategies for self-harm and suicide behaviours may be introduced early and may promote social networks as a key source of support. This review found a paucity of peer-reviewed research within some populations. More peer-reviewed research is needed, particularly among these populations where current work is limited, and regarding modifiable risk and protective factors.


Introduction
Self-harm and suicide behaviours are a major public health concern, with over 700,000 people dying by suicide and an estimated 14.6 million people engaging in self-harm each year globally [1,2].The aetiology and onset of these behaviours is complex and multifaceted; prevalence and risk are influenced by several factors including age, sex, ethnicity, geographical region and occupation [3][4][5].Military populations are a potentially at-risk group as military service impacts on the health and wellbeing of personnel during and after service [6].Self-harm and death by suicide appear to be relatively rare among the military community, although rates of both behaviours have increased in recent years but remain either lower or comparable to the general population [7][8][9][10][11][12].Suicide behaviours can present as ideation (i.e.thoughts) and attempts which are also prevalent among serving and ex-serving personnel; a reported 11% global prevalence of these behaviours in the entire military [13], with prevalence varying slightly between serving and ex-serving personnel for both suicidal ideation (serving = 10%; ex-serving = 14%) and suicide attempts (serving = 8%; ex-serving = 15%) [13].
Previous research has reported that serving and ex-serving personnel with mental health diagnoses are at risk of self-harm and suicide behaviours, for example post-traumatic stress disorder (PTSD) [14][15][16][17][18]. Another specific at-risk group includes those separated from the military [19][20][21], especially if separation occurred involuntarily (e.g.medical discharge) [22].Additionally, younger age is a risk factor [22][23][24]; a recent UK study reported suicide risk was two-to-three times higher in men and women under 25 years old who had left service (vs the general population) [24].Ex-serving personnel with a shorter length of service (<10 years of service) were also at increased risk [24].Protective factors have been less frequently explored, but include social support (i.e. the availability and adequacy of social connections [25]) [26,27], higher educational attainment [28], employment [29] and holding a more senior rank (e.g.officer) [15,18].
There remains a paucity of research exploring the risk and protective factors for self-harm and suicide behaviours among several military populations, with available literature focussing on a limited number of countries.This review focused on military populations from four nations of the Five Eyes intelligence alliance [30]: namely the UK, Canada, Australia and New Zealand.The Five Eyes nations (which also includes the United States (US)) have a combined military population estimated at 2.6 million, are all developed countries which share a common language, have similarities in society and culture, broadly similar military involvement, and well-resourced military healthcare systems [31].Of the limited systematic reviews in the field, studies on US military populations make up all or the majority of included papers [27,32,33].To address this gap in the literature, the current review focuses on the remaining four Five Eyes nations where attention has been limited previously.Additionally, the US was excluded due to differences in firearms culture, including access, licensing and laws which affects access to means of suicide; a recent systematic review highlighted that US veterans are at substantially increased risk of firearm suicide and have higher rates of firearm ownership than the US general population [34].
The current systematic review aimed to identify risk and protective factors associated with self-harm and suicide behaviours among serving and ex-serving personnel of the UK Armed Forces, Canadian Armed Forces, Australian Defence Force and New Zealand Defence Force.

Design
This systematic review was conducted following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [35].Prior to commencing the review, the protocol was registered with PROSPERO (CRD42022348867), an online database of systematic review protocols submitted prospectively to maintain research integrity.

Search strategy
Seven electronic databases were searched in October 2022: PubMed (including MEDLINE and PubMed Central), Web of Science, Embase, Global Health, PsycINFO, PTSDpubs and CINAHL.All databases were searched using pre-defined terms related to: (1) self-harm and suicide behaviours, (2) the military, and (3) geographical locations.See S1 Table in S1 File for full search strategy.
The search included all original, peer-reviewed work that identified risk and/or protective factors associated with self-harm and/or suicide behaviours both during (serving personnel) and after (ex-serving personnel/veterans) military service.Restrictions were placed on publication dates from 1 st January 2001 to 30 th September 2022 to allow for the coverage of the start of the Iraq and Afghanistan conflicts.Additionally, these limits were chosen due to better availability and quality of literature in the field of military medicine since 2001 [36].
PROSPERO was searched to identify any ongoing systematic reviews and meta-analyses of relevance.Additionally, the reference lists of included studies and other relevant studies were searched, including identified systematic reviews and meta-analyses of relevance.If the fulltext of a paper was not available online, authors were contacted for access.At least one expert in the field of military self-harm and/or suicide behaviour research from each nation was contacted to ensure key papers had been identified.Experts were identified via the Five Eyes Mental Health Research and Innovation Collaboration [37].

Study selection criteria
Eligibility was determined using the following criteria: • Published in English • Original, peer-reviewed work • Published 1 st January 2001 to 30 th September 2022 • Studies that reported risk and/or protective factors for outcomes of self-harm, suicidal ideation, suicide attempts and/or completed suicide • Population comprising of serving and/or ex-serving personnel (Regulars/Reservists; Navy/ Army/Air Force/Marines) from the UK Armed Forces, Canadian Armed Forces, Australian Defence Force or New Zealand Defence Force • If military only made up part of the reported sample, only papers that reported on outcomes for serving/ex-serving personnel separate from the other population (e.g., general population) were included It is important to note the term 'veteran' is defined and used differently across nations.For instance, in the UK a veteran is defined as someone who has served a minimum of one day paid employment in the UK Armed Forces but no longer serves [38], this can include those who deployed or not and is irrespective of their type of discharge.However, in Canada, a veteran is any former member of the Canadian Armed Forces who successfully underwent basic training and is honourably discharged [39].Therefore, for the purpose of this review, we adopted the term 'ex-serving personnel' to mean someone who served in the Armed Forces but no longer serves, irrespective of deployment experience or type of discharge.
Exclusion criteria included: • Study design: qualitative, systematic reviews/meta-analyses, pilot studies, case-control studies, case reports/series, study protocols, grey literature, conference abstracts/papers, dissertations/theses, and editorials • Outcomes of assisted suicide (i.e. the act of deliberately assisting another person to kill themselves [40]) and suicide bombing (i.e. a terrorist bomb attack in which the perpetrator expects to die whilst killing a number of other people) • Studies that only included a population sample of conscripts (i.e., people compulsorily enlisted into the military), cadets or officer students

Screening and data extraction
Following the initial search, all identified studies were imported into Endnote 20 and duplicates were removed.CW independently reviewed the titles/abstracts of all papers.Subsequently, the full papers for the remaining studies identified as relevant were then reviewed.
The reference lists of all included papers were manually searched for any additional papers of relevance (i.e., cross-referencing).BC independently performed second reviewer screening on a sample of studies (10% at title/abstract screening stage and 20% at full text screening stage).
The reviewers (CW and BC) independently decided which studies met the eligibility criteria to be included in the review and, at full text screening stage, noted any reasons for exclusion.Any discrepancies were resolved through discussion.Interrater reliability was calculated at each screening stage; agreement was 99% at title/abstract and 100% at full-text stage.
The following data were extracted independently by CW for all included papers where available: • General information: title, lead author, publication date, journal title, location/country of study

Data synthesis
To examine the risk and protective factors associated with self-harm and suicide behaviours, a narrative synthesis was performed.This approach was chosen due to heterogeneity across studies, for example, variation in sample sizes, target populations and outcomes [41].Due to variation across included studies, such as differences in study design and outcome measurements, and the large number of risk and protective factors identified, it was neither practical nor feasible to conduct a meta-analysis as part of this review.

Quality assessment
The quality of each included paper was assessed independently by CW using the National Heart, Blood and Lung Institute (NHBLI) tailored quality assessment tools [42].BC independently performed second reviewer quality assessment on a sample of papers (20%).Any discrepancies were resolved through discussion.Papers were not excluded based on their quality, instead, results from the assessment provided additional insight into the quality of research in the field.

Results
Overall, 4,576 papers were identified, of which 497 duplicates were removed (Fig 1).The title/ abstracts of 4,079 papers were screened, leaving 94 papers at full-text stage.Experts in the field identified no additional peer-reviewed papers and cross-referencing identified one additional paper of relevance.Overall, 28 papers met the inclusion criteria (S3 Table in S1 File).

3.2.Quality assessment
Most papers received a quality assessment score of 'fair' (n = 21), and the rest scored 'good' (n = 7) (S4   justification/power description, not assessing the exposure variables more than once, and not using outcome measures which were clearly defined, valid and reliable.

3.5.Risk factors
Numerous risk and protective factors were identified in the review with varying levels of statistical significance (Table 2).

Risk factors for self-harm.
Pre-enlistment factors increased the risk of self-harm, including experiencing childhood adversity and abuse than those without these experiences (approximately doubling the risk) [15,60].Certain demographic groups were also at heightened risk, including younger age groups [15,60,61] compared to middle/older age groups, and those no longer in a relationship (i.e., separated/divorced) [60] compared to in a relationship.There were discrepancies between studies regarding the impact of sex, with two studies reporting the risk of self-harm was increased for females [18,60] but one study stated increased risk for males [62].Military characteristics associated with increased risk of self-harm included having left service, with ex-serving personnel reported to be as much as three times more at risk of self-harm than serving personnel or non-military population depending on sample explored [15,18,60,62].Other military characteristics that increased the risk of self-harm included being an early service leaver, defined within the relevant papers as <4 years [54] and �2.5 years [62] of service.This was also apparent for those with a shorter length of service (but not an early service leaver), for instance 4 to 6 years of service [62].
Several health-related factors also increased risk of self-harm.The factors with some of the largest effect sizes (ranging from approximately two to eight times more likely) included clinical or probable diagnosis of PTSD [15,18,62], depression [15,18], and anxiety [18,62], as well as history of suicidal ideation [18].Other risk factors for self-harm included a lack of social support [60], mental health-related stigmatisation [18], perceived practical barriers to care  https://doi.org/10.1371/journal.pone.0299239.t002[18], negative attitudes to mental illness [18], and formal medical help-seeking [18] (all approximately tripling risk).3.5.2.Risk factors for suicide behaviours.Similar risk factors were identified for suicide behaviours (i.e., suicidal ideation, suicide attempts and completed suicide).Pre-enlistment factors, such as experience of childhood adversity and abuse were found to increase the risk of suicide compared to those without these experiences (ranging from two to seven times more likely) [48][49][50]56,59].Certain demographic characteristics increased the risk of suicide behaviours, including being single or no longer in a relationship (i.e., separated, divorced, widowed) [45,48,51,55], lower educational attainment (i.e., less than university degree level) [48], and unemployment [29,59].Another identified risk factor for suicide behaviours was a lack of social support [43].The influence of age remains unclear, as studies reported a variety of age groups to be at increased risk of suicide behaviours [43,48,63,64,66]; for instance younger age groups (e.g., 16 to 24 years [64]), younger/middle age groups (e.g., <25 to 44 years [66]), and a mixture of age groups (i.e., 25 to 45+ years [48]).
Numerous military characteristics increased the risk of suicide behaviours, including being medically discharged from service [55], holding a more junior rank during service [48,49,66] (e.g., non-commissioned officer or other lower ranks [48,49]), serving as a Reservist (vs Regular) [18], and shorter intervals since return from last Afghanistan deployment (i.e., <4 years) [58].Being an early service leaver was another factor that increased risk of suicide behaviours, defined within the relevant papers as �3 years [63] and <4 years [54,59] of service (e.g., increased risk by as much as eight-fold [59]).Although not referred to as early service leavers, having a shorter length of service (i.e. one to four years) was also a risk factor for completed suicide among a sample of UK ex-serving personnel [64].Experience of deployment-related traumatic events [44,46,47,50,[56][57][58] and exposure to trauma [44,48,49,53,57] were other factors associated with increased risk of suicide behaviours.

3.6.Protective factors
A number of protective factors were also identified (Table 2).
Several military characteristics decreased the risk of suicide behaviours, including holding a more senior rank (vs junior ranks) [18,64] (e.g., officer rank [64]), service in the Royal Air Force [18,64] or Naval Services [64] (vs Army), and certain deployment-related factors such as higher cumulative duration of Afghanistan-related deployments (i.e., �361 days) [58], and less time away on deployment in the past three years (i.e., up to two years) [58] (all approximately half as likely).Higher levels of perceived social support [18,44] (e.g., a score of 33-36 on the multidimensional scale of perceived social support [18]), and taking longer than five years to seek support (vs less than five years) [59] were also identified factors that reduced the likelihood of suicide behaviours.

Discussion
This review identified 28 papers reporting on a range of factors associated with self-harm and suicide behaviours among serving and ex-serving personnel of the UK Armed Forces, Canadian Armed Forces, and Australian Defence Force.
A variety of definitions and measurements were used for the outcomes reported in the included papers.Definition is an important aspect of academic research and clinical practice, yet precise definitions have been contested, and current terminology varies across nations [68].For instance, there are several terms for self-harm, including non-suicidal self-injury, deliberate self-harm and self-inflicted violence.Definitions of mental health conditions and the use of consistent language are an important starting point within good quality research papers and are also important for reducing stigma and encouraging individuals to seek help, particularly relating to self-harm and suicide behaviours [69].
Several of the identified risk factors reflect those among the general population [3,70], serving and ex-serving military personnel of the US Armed Forces [27,33], and other similar occupational groups (such as emergency responders) [71], and were generally consistent across the included geographical regions.Some similar identified risk factors for suicide among the US military community include mental health diagnoses (e.g.mood, alcohol/substance, psychotic and personality disorders), and experience of childhood adversities [27].
Although no relevant peer-reviewed papers were identified from New Zealand, suicide prevention work is being conducted [72].Risk factors for suicide among the New Zealand Defence Force include current mental health concerns, acute life stressors and negative attitudes towards help-seeking, and protective factors include positive mental health and social support [72].The paucity of peer-reviewed work from New Zealand may be due to the smaller military population of the New Zealand Defence Force compared to the military populations of other included nations, making large quantitative studies more difficult.
It is important to recognise that some risk and protective factors were not always consistent across included studies in this review, and the influence of age, sex, certain military service characteristics, and certain health diagnoses remains unclear.For instance, having served as a Reservist increased the likelihood of suicide behaviours [18] but decreased the likelihood of self-harm [60], highlighting the need for a holistic approach when supporting military communities.It is important to develop an enhanced understanding about motivations for engaging in self-harm and suicide behaviours, as for some self-harm acts as a coping mechanism to regulate internal emotions, whereas suicide behaviours become more apparent when the individual can no longer cope [73].
One key risk factor for self-harm was having left service.Potentially the transition from serving to ex-serving is a period of risk and support should be in place during this time.It might be that after leaving service, ex-serving personnel no longer feel like part of the military 'family', experience a shift in their sense of self and have difficulty connecting to civilian life [74].Alternatively, it could reflect the influence of time as ex-serving personnel are typically older, and therefore, if considering lifetime prevalence and risk, this would capture a longer period of time.Additionally, there may be an underrepresentation among serving personnel as they are potentially more hesitant to report these behaviours due to stigma and fear of negative consequences to their career [75]; personnel with ill health (physical or mental) may be forced to leave service (i.e.medical discharge) and those who involuntarily separate from the military are at high risk of self-harm and suicide behaviours [22,76,77].
Poor health was another key identified risk factor, in particular clinical or probable diagnosis of PTSD, depression and anxiety.This highlights the importance of early detection of mental health problems, providing adequate care and support to military personnel throughout their military career, and providing continuity of care as they transition out of service.Interestingly, seeking help from formal medical sources was positively associated with lifetime suicide attempts [18] and taking more than five years to seek help was negatively associated with suicidal ideation [59].It is unlikely that seeking help from formal medical sources has a causal relationship with suicide behaviours.Instead, UK research has found that ex-serving personnel are known to present to clinical services at times of crisis, which might involve an active episode of self-harm or suicide behaviours, therefore, placing them more at risk [78].Additionally, those who delay seeking help for longer may have discovered ways to cope with their difficulties on their own, whereas those who seek help sooner may have been in crisis and at higher risk of suicidal ideation [59].Clearly, a key challenge relating to self-harm and suicide behaviour risk is prevention.Most developed nations already have suicide prevention strategies in place, including for serving and ex-serving personnel, or as part of the wider mental health strategy [79][80][81][82].The findings of this review suggest that prevention and intervention strategies should focus on the broader context of improving health and wellbeing (physical, mental, and social health).
Less research focussed on protective factors for self-harm and suicide behaviours among the included military populations.Despite this, one key association was with higher levels of perceived social support [18,44], indicating that this was likely a salient factor that could help prevent or mitigate risk, particularly due to its modifiable nature.This aligns with other international military populations, including the US [83], as well as in the general population where social support has been recommended for use in suicide prevention strategies [84].One way to bolster social support among military populations is the use of peer support as a preventative strategy [85,86].

Strengths and limitations
A strength of this review was the search of seven literature databases using a broad search strategy outlined in an a priori PROSPERO approved review protocol.The protocol was generally followed but any changes were reflected by updating the protocol (i.e., removing the limit around the age of the sample).Additionally, a proportion of the eligibility assessment and critical appraisal of papers was conducted by a second, independent reviewer with high inter-rater reliability.Including a wider range of geographical regions may have led to more included papers (e.g., Na et al (2021) [87]), however focus this review aimed to address the literature gap by collating evidence where attention has previously been limited.
There were some limitations to note.As with all systematic reviews, the findings of this review were subject to publication bias.Additionally, some potential associated factors received less attention than others, however this review tried to provide the best synthesis of the evidence currently available.
There were also several limitations relating to the included papers.First, papers used different definitions and measurements of self-harm and suicide behaviours, limiting the possibility of comparing findings across studies.However, combining these papers in a review contributes to understanding.Second, there is no universally agreed definition of veterans (i.e., exserving personnel) which makes cross-cultural comparison difficult.Third, studies did not always report on some military characteristics (such as service branch, rank, deployment experience, era of service and time since leaving service) and demographic characteristics (such as ethnicity and sexuality) which would have been useful for contextualisation and interpretation of the findings.There remains limited understanding of the impact of sex, experiences of ethnic minority and LGBTQ+ personnel.Fourth, included studies used a variety of sample sizes (range n = 144 to 233,803), study procedures, and sample characteristics.For example, the majority of papers relied on self-reported data which may have been subject to recall bias and social desirability bias.Finally, the included papers did not allow for meta-analyses to be conducted due to heterogeneity in the populations and outcomes.

Implications
This review has several important implications for policy, practice, and research.The identification of risk and protective factors can be useful to inform military health services and policies including where to target suicide prevention policy to reduce the incidence and impact of suicide.In the UK, this is one of nine key health priority themes laid out in the Defence People Health and Wellbeing Strategy 2022-2027 [88].
Identifying the risk and protective factors for self-harm and suicide behaviour outcomes is an important aspect of the development and implementation of effective prevention and intervention strategies to protect the mental health and wellbeing of military populations.Evidence on associated factors can inform healthcare practice and service provision for the Armed Forces community by highlighting several at-risk groups which may require additional support.This review suggests that additional support is required during the period of transition from military to civilian life but also highlights the importance of prevention early on in military service to reduce the impact on personnel as they transition out of service.The identified protective factors suggest that prevention and intervention strategies should focus on encouraging help-seeking for mental health problems early on before crisis events occur, as well as promoting social support networks and strengthening connections with family, friends and the community as a whole.
Additionally, focus should be placed on modifiable factors (i.e., factors that could reasonably be altered, such as psychiatric symptoms and social support).Although non-modifiable factors can help to identify level of risk, they are of less use during prevention and intervention as they cannot be changed to alter the level of risk.Therefore, particular focus on modifiable risk and protective factors is warranted as they are amenable to therapeutic intervention and can be key to addressing long-term risk due to their adaptive nature.However, it is still important to understand the role of non-modifiable factors as there may be an indirect effect, for example when considering military rank, it might be that officers are at lower risk of self-harm and suicide behaviours because they typically hold higher socio-economic status and higher educational attainment [89] which are known protective factors.
Future studies should also focus on conducting longitudinal investigations which distinguish between pre-, peri-and post-service factors in order to identify pathways of self-harm and suicide behaviours, and to ensure support is in place at the right point in the military lifecycle.

Conclusions
This review highlighted several risk and protective factors for self-harm and suicide behaviours which warrant attention.Adequate care and support are a necessity for serving and ex-serving personnel who may be at risk of experiencing self-harm or suicide behaviours.Particular focus should be placed on implementing prevention strategies early on in military service to reduce the impact on personnel as they transition out of service.The identified protective factors suggest that prevention and intervention strategies should promote social networks as a key source of support for military personnel.Whilst this review was limited due to the paucity of peer-reviewed research within some populations, current work, such as that being undertaken in New Zealand will add to the understanding.Research should continue to progress towards understanding and preventing self-harm and suicide behaviours among military populations.

Note.
Significance refers to adjusted effect sizes where available.If only unadjusted effect sizes are presented, studies have been marked by an asterisk (*).ESL: Early service leaver; DRTE: Deployment-related traumatic event; RAF: Royal Air Force; CF: Canadian Forces; NCO: Non-commissioned officer; NCM: Noncommissioned member; DSM: Diagnostic and Statistical Manual of Mental Disorders; PTSD: Post-traumatic stress disorder; MDD: Major depressive disorder; DAR-5: Dimensions of Anger Reactions 5-item; PCL-5: PTSD Checklist for DSM-5; PHQ-9; Patient Health Questionnaire; AUDIT: Alcohol Use Disorder Identification Test.
Where available, adjusted odds ratios or effect estimates have been presented.Otherwise unadjusted results have been reported and clearly identified.
• Study characteristics: study aim, study design and methods, response rate (where relevant), sample size, data collection date • Sample characteristics: age in years (mean, median or range), sex distribution, ethnicity, population type (e.g., clinical/non-clinical) • Military characteristics: serving status (serving/ex-serving), engagement type (regular/ reserve), service branch (Army, Naval Services, Air Force), rank (other, non-commissioned, commissioned), deployment experience (e.g., number and duration of deployments), era of service (e.g., Iraq and Afghanistan era) • Outcomes: outcome type (i.e., self-harm, suicidal ideation, suicide attempts, completed suicide), definition of outcome, how outcome was measured • Risk and protective factors associated with self-harm and/or suicide behaviour outcomes • Study findings: conclusions, limitations, future research Statistical findings are reported in S2 Table in S1 File.

Table 1 .
(Continued) +1 use data from the Canadian Forces Mental Health Survey; +2 use data from the Canadian Community Health Survey: Mental Health and Well-Being Canadian Forces Supplement (CCHS-CFS); +3 use data from the Scottish Veterans Health Study; +4 use data from various phases of the King's Centre for Military Health Research (KCMHR) Health and Wellbeing Study; NCO: Non-commissioned officer; NCM: Non-commissioned member; ICD: International Classification of Diseases; NHS: National Health Service (UK).https://doi.org/10.1371/journal.pone.0299239.t001